astr 330: the solar system announcements dr conor nixon fall 2006 homework assignment #5 on-line

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Fall 2006

• Homework assignment #5 on-line.


Lecture 21:

Large Satellites I: Callisto, Ganymede and Europa

Dr Conor Nixon Fall 2006Picture credit: NASA/JPL - Galileo


From planets to moons


• We have now completed out tour of the nine ‘official’ planets of the solar system.

• We now turn our attention to the large moons of the outer planets by which we mean:

• The four ‘Galilean’ moons of Jupiter: Io, Europa, Ganymede and Callisto.

• Saturn’s giant moon Titan, the only moon with a substantial atmosphere.

• Neptune’s large moon, Triton.

• Each one of these large moons would have a significant claim to being a planet in its own right, had it been orbiting the Sun rather than a planet.

• Note that only Uranus, of the four ‘giant’ planets, lacks a 3000-5000 km sized moon. The two largest, Titania and Oberon are around 1500 km.


By the numbers


• Facts and figures for the six large moons, and also Mercury and Pluto are shown in the table below.

Table: Morrison and Owen

Satellite Period Diameter Mass Density(days) (Moon=1) (Moon=1) (g/cm3)

Callisto 16.69 1.38 1.47 1.9

Ganymede 1.16 1.51 2.02 1.9

Europa 3.55 0.91 0.65 3.0

Io 1.77 1.04 1.22 3.6

Titan 15.95 1.48 2.00 1.9

Triton 5.88 0.84 0.30 2.1

Mercury 1.40 4.49 5.4

Pluto 0.66 0.17 2.1

• Which one is the:

• largest?

• most massive?

• densest?

• Which moons are bigger than:

• Mercury?

• Pluto?


Cratering On Icy Moons


• Most of the large moons (those with densities around 2 g/cm3) are roughly equal mixtures of rock and ices.

• These moons have differentiated internally to varying amounts, but in general the surfaces are quite icy.

• What then is the implication for the cratering record? On the terrestrial planets there was no doubt that cratering of rocky surfaces was capable of retaining crater imprints over billions of years, if left in peace.

• But can icy surfaces retain crater records?

• The answer depends on temperature. On the Earth, ice is clearly quite plastic and flows down mountains in the form of glaciers.

• At Saturn, water ice is frozen as hard as rock. However, at the distance of Jupiter, the ice may not only deform and lose shape, but may also sublime and evaporate.


Impacts in the Outer Solar System


• For the Earth and Moon, 75-90% of craters are thought to have been caused by asteroid impacts.

• However, few asteroids reach Jupiter, and virtually none can reach Saturn, so we expect asteroid craters rates to be pretty insignificant.

• Jupiter’s gravity partly shields the inner solar system from comets, so we expect more cometary impacts in the outer solar system.

• At Jupiter, most impacts come from ‘Jupiter family’ comets: objects diverted from the Kuiper Belt.

• At Saturn, most impacts come from long-period comets, from the Oort Cloud.

• The impact rates at Jupiter are about half that of the Earth, at Saturn the rate is one quarter the Earth rate.


Influence of Gravity on Cratering


• There is an important correction we must make when comparing cratering on different satellites.

• The gravity of the parent planet tends to pull comets inwards: the more massive the planet, the stronger the gravitational effect.

• We can imagine the planets as situated at the bottom of a slope: the closer the moon is to the planet:

1. the more impacts will occur, and2. the higher the impacts speeds will be.

• For example: Mimas, the innermost large moon of Saturn would expect 20 times the impact rate of Iapetus, far from the planet.

• Inner moons of Jupiter and Saturn therefore have higher cratering rates than the Earth and Moon, whereas distant moons would have a lesser rate.


Cratering Rates Over Time


• We have just seen that the gravitational focusing of the parent planet can compensate for the lower number of potential impactors, so that the cratering rates are not too different for bodies in the outer solar system, compared to the inner solar system.

• In fact, we do see crater densities not much different from the Moon, which we have noted are in contradiction to the number of available impactors at the present day.

• Hence, cratering rates were higher in the past than now, and we are led to the conclusion that the ‘Late Heavy Bombardment’ period we surmised in the inner solar system also applied to the outer solar system.


Galilean Satellites Of Jupiter


• The composite image above shows the four large Galilean satellites of Jupiter to scale. From left to right they are: Ganymede, Callisto, Io and Europa. While Ganymede is larger than Mercury, Europa is slighter smaller than the Moon.

• The moons are all named after ‘conquests’ of Jupiter/Zeus in mythology.

Picture credit: NASA/JPL - Galileo


Callisto

Dr Conor Nixon Fall 2006Picture credit: NASA/JPL Voyager 2

• The image (right) was taken by Voyager 2.

• What can you tell from this picture about the:

• atmosphere?

• surface composition?

• geology or history of the surface?


Callisto Interior


• Callisto is about the same size as Mercury, but only 1/3 as massive. Hence, it is about half water ice and half rock, rather than rock and iron.

• Callisto, like all the large satellites, should have been able to fully differentiate, due to heating from radioactive elements in the rocky component. This would imply a rock-mud core and an icy mantle and crust.

• However, gravitational field measurements from Galileo showed that the planet does not have a big change in density towards the core, and has therefore not fully differentiated.

• Additionally, a weak magnetic field was detected by Galileo, which may be explained by a salty fluid below the surface: a possible ocean layer.


Model Callisto Interior


• A model Callisto interior is shown right.

• The outer layer is a 200 km thick ice crust with a 10-km ocean overlying the interior mantle.

• The mantle is a mostly homogeneous mixture of rock and ice.

Picture credit: solarviews.com


Callisto Surface


• The temperature of Callisto varies from about 100 K at noon to 150 K at night, low enough for ice to evaporate extremely slowly: a few meters over the last 4 billion years.

• Ice is observed on the surface of Callisto (by spectroscopy), but the reflectivity of just 0.18 shows the ice is rather dirty, perhaps mixed with meteoric dust.

• As expected for a world with almost no internal activity, the surface retains an ancient cratering record, with a density almost as great as the lunar highlands: 250 10-km craters per million square km.

• Hence, we can safely say that the surface of Callisto appears at least as old as the lunar maria, and probably formed during a similar period of Heavy Bombardment.


Cratering


• On close examination, the craters are different shapes to those of the Moon. Instead of being bowl-shaped, they are flattened.

• This is because at Callisto’s temperature, the ice is not perfectly solid, and flows very slowly over time, gradually smoothing the initially sharp crater forms over hundreds of millions of years.

Picture credit: NASA/JPL Voyager 2

• The image (right) from Voyager 2 shows the Gipul Catena, a crater chain 620 km long.

• Features such as this mystified scientists until the S-L 9 break-up was observed.


Giant Impacts


• Callisto apparently has suffered similar giant impacts to those which produced basins such as Imbrium on the Moon and Caloris on Mercury.

• However, on Callisto we see only the concentric rings, not the basin itself. Presumably, ice is too plastic to retain the sunken basin shape.

• The Vahalla impact (right) is 3000 km across, and a similar bulls-eye pattern occurs at the Asgard impact, which is 1700 km across.

Picture credit: Bill Arnett/Nine Planets


Small-scale topogrpahy


• At the smallest scales of just 10s of meters, other distinctive features become apparent.

Picture credit: NASA/JPL Galileo

• The Galileo image (right) shows a region just 13 km across, with the smallest visible craters here 130 meters in size.

• A process of evaporation and condensation has taken place here. Ice has evaporated from some regions, leaving behind dark surface material.

• Some of the ice has re-condensed on cooler slopes which face away from sunlight.


Ganymede


• Ganymede is the largest moon in the solar system, and the same density as Callisto.

• The large dark region, called the Galileo Regio, is about 3200 km in diameter.

• Parts of this terrain may be covered in a bright frost.

Picture credit: NASA/JPL


Ganymede Interior

Dr Conor Nixon Fall 2006Picture credit: solarviews.com

• Ganymede, unlike Callisto is definitely differentiated, as tracking of the Galileo spacecraft proved.

• Ganymede has a large, dense core, presumably of silicates, surrounded by an icy mantle and crust.


Ganymede Surface


• Ganymede’s surface may be divided into two categories:

1. About half the surface resembles the ancient, dark terrain of Callisto, with crater ghosts and concentric ridges from long-ago impacts. This terrain has a similar level of cratering, and hence age, as Callisto. The albedo is about 25%, suggesting a relatively low admixture of ice with the ‘dirt’.

2. The rest of Ganymede is a lighter colored, less-cratered type of terrain. The densities here are 100-200 10-km craters per 1 million km2, indicating an age of 1-2 billion years. The albedo is 40%, indicating a higher proportion of ice.

• Note that the age/brightness difference is the opposite to the Moon, where the darker areas (the maria) are younger than the lighter highlands.


Indications of Internal Activity

Dr Conor Nixon Fall 2006Picture credit: NASA/Galileo/Nine Planets Page

• Many of the younger areas of Ganymede show parallel ridges and valleys, separated by 10-15 km. These are similar to some areas of the Appalachians.

• Although these are tectonic features, they are not the same as the fold mountains we see on the Earth.

• Rather than being produced by a horizontal compression, these features are believed to be parallel cracks produced by uplift and subsidence: vertical motions.


Craters


• Ganymede has a small number of very bright craters, suggesting that nearly pure water ice has been ‘splashed’ across the surface by an impact.

• Callisto does not show similarly bright crater ejecta, suggesting that the sub-surface ice on Ganymede is much cleaner: further evidence for greater differentiation.


• This Galileo image shows yet another crater chain (the Enki Catena): notice that the impact has occurred right across the boundary between light and dark terrains.


History of Ganymede


• The evidence we have accumulated so far seems to indicate that Ganymede suffered a series of internal upheavals sometime in the first 2 billion years of its existence.

• One possible cause could be a change of density and structure of internal ices, as the moon gradually cooled since formation.

• This may have caused a series of expansions and contractions which could crack the surface and flood some of the valleys with water ‘lava’, explaining the flattened floors of some valleys.

• Magnetic field measurements indicate a weak magnetic field, but there is a possibility of a liquid ‘slush’ layer deep in the mantle which is electrically conducting (similar to Callisto).


Europa


• Below: natural (left) and false color (right) images of Europa’s trailing hemisphere, from the Galileo spacecraft.

Picture credit: NASA/JPL - Galileo


Europa: Overview


• Europa is the smallest of the ‘big four’ Galilean moons of Jupiter.

• Europa’s density of 3 g/cm3 indicates a much smaller amount of ice in the compositional mixture (10%) than either Ganymede or Callisto.

• However, the surface is the brightest of the four (reflectivity 70%) and appears to be nearly pure water ice. In this respect, Europa resembles what the Earth would be like, if all the oceans were frozen.

• The surface ice appears to comparatively pure at the present day, unlike the ‘dirty ice’ appearance of Ganymede and Callisto.

• Europa is the smoothest planetary object in the solar system, with almost no impact craters visible at all, despite the ‘focusing’ effect of its closeness to Jupiter. Any record of historical bombardment has been erased. How?

• The surface age is thought to be just 10 Myr: younger than the Earth’s surface!


Linear Markings


• Europa’s most remarkable features are the many linear markings: similar to the fanciful drawings of Martian canals by Lowell .

• The markings are in fact tectonic cracks produced by stresses in the crust, which can stretch for 100s or 1000s or km.

• The implication is that the crust is floating on a layer of liquid water or slush.

• The tectonic cracks are often a ‘double ridge’ type, a few km in width, and several hundred meters in height.

• Scientists suspect that these may have formed when slushy material was injected into a crack from below, or due to pressure at the closing of the crack.

• We see similar pressure ridges in the Earth’s Arctic ice pack, but on a much smaller scale.


‘Double Ridge’ Cracks

Dr Conor Nixon Fall 2006Picture credit: NASA/JPL Galileo


Scalloped Ridges


• Even more remarkable are the ‘scalloped’ ridges, which show multiple arcs along their length. These are believed to have been caused by successive daily tides.

• Each Europan day, the crack would form in an arc, due to the changing direction of tidal forces. The next day (3.5 Earth days), another arc is created, adding to the first.



Europa: Chaos Terrain


• A few percent of Europa’s surface is classified as ‘chaos’ terrain (remember the chaos terrains of Mars?).


• These areas show pieces of older crust, criss-crossed with many ridges, which have apparently rotated and drifted into new positions, even tilted, and then been re-frozen back into the solid crust again (like a mixed up jigsaw puzzle).

• Periodically, the crust must melt, allowing liquid water to reach the surface.


Europa Interior


• The idea of a global ocean on Europa is the most certain of the three candidate moons of Jupiter (Callisto, Ganymede, Europa).

Picture credit: solarviews.com

• Aside from the chaos terrain, magnetic field measurements indicate a conducting fluid (e.g. salty water) is present sub-surface.

• The crust is probably 10-20 km thick, and the water ocean is the largest in the solar system, Earth included!


Quiz-Summary


1. Should large moons have differentiated at all? What factors sould affect the amount of differentiation in a given moon?

2. Name the largest moon of (i) Jupiter (ii) Saturn (iii) Neptune.

3. Why might the cratering record be preserved less well on the surfaces of the outer planet moons, than say the Moon or Mercury?

4. Would we expect the same populations of objects to cause craters on the outer planet moons, as on the inner planets? Why?

5. What effect does the parent planet have on the amount of impactors reaching (I) a close-in moon such as Mimas (ii) a far-out moon such as Iapetus?

6. Was there a period of heavy bombardment in the outer solar system?


Quiz-Summary


7. Callisto and Ganymede are similar sizes and densities. Are they the same inside?

8. What differences are there between craters on Ganymede and Callisto, and those on Mercury and the Moon?

9. What two types of surface terrain do we see on Ganymede? Which is also seen on Callisto? Which is older?

10. Which is more like a billiard ball: Europa or Callisto?

11. What causes the linear double-ridges on Callisto?

12. Europa’s scalloped ridges are caused by tides; what length of time does each scallop shape correspond to?

13. Which Galilean moon is most likely to have a liquid ocean? How much, e.g. relative to the Earth’s oceans?

astr 330: the solar system announcements dr conor nixon fall 2006 homework assignment #5 on-line

Documents

solar system cratering

solar system impacts

inner solar system

solar system lecture

large moons

icy moons dr conor nixon

cratering dr conor nixon

europa dr conor nixon